1. Field of the Invention
The present invention relates to a method of determining the digging position of a digging head in a substantially horizontal digging or excavation and, more particularly, to a method that ensures accuracy in determining positions of the digging head by lessening the influence of noise magnetic field frequency components close to the signal magnetic field to be measured.
2. Brief Description of the Prior Art
A horizontal drilling method, which is one of the boring or trenching methods of this kind, uses a small-diameter pipe of 100 mm or less for horizontally digging in the ground, and, accordingly, the kind of precision position-determining apparatus used in an ordinary small-diameter driving method of excavation cannot be placed near a drill. To solve this problem, it is customary in the art to generate an AC magnetic field by means a coil mounted in the drill head that is detected by an above-ground magnetic sensor like a coil to determine the current digging position.
This method is simple and easy, but since the magnetic field by the coil is a dipolar magnetic field, the field rapidly attenuates with distance from the coil. The method is defective in that it is unable to achieve a highly reliable determination of the digging position when a power line or similar magnetic noise source is present in or around the place where the position determination is to be performed.